Multi-point fuel cell voltage monitor

ABSTRACT

A fuel cell voltage monitor ( 12   a   , 12   b   , 40, 140, 440 ) detects and may respond to, a problematic operating condition at or near a fuel cell ( 10 ) or within a subset (n) of fuel cells, as in a fuel cell stack assembly ( 110 ). Two or more co-planar, spaced voltage leads or contacts in a fuel cell plane, as at a separator plate, detect the presence of a voltage difference within the plate/plane as an indication of an operating problem at or near the fuel cell. Placement of such arrangements of at least two spaced voltage leads at various subset intervals (n), of fuel cells in a stack assembly allow monitoring for such problems throughout the stack assembly, either by analysis of voltage difference between co-planar leads at respective individual fuel cells or by comparison of voltage differences between aligned pairs of voltage leads at opposite ends of a subset.

TECHNICAL FIELD

This disclosure relates to fuel cells and more particularly to monitorsfor fuel cells. More particularly still, the disclosure relates to afuel cell voltage monitoring arrangement.

BACKGROUND ART

Fuel cells provide clean, direct current electricity by convertingreactants, namely fuel (typically hydrogen) and oxidant (air or oxygen),to generate electric power and reaction products. Fuel cells generallyemploy an electrolyte disposed between two electrodes, namely a cathodeand an anode. A catalyst typically induces the desired chemicalreactions. The catalysts are typically located at the interfaces betweeneach electrode and the adjacent electrolyte.

Fuel cells having various types of electrolytes are within the scope ofconsideration. Some fuel cells have solid polymer and solid oxideelectrolytes, whereas others employ liquid electrolytes, etc. Fuel cellassemblies require separator plates for providing current passage andseparating the fuel and oxidant reactant gases between the cells. Theyare disposed on each side of the electrode/electrolyte assembly of theindividual fuel cell.

During operation, the output voltage of an individual fuel cell underload is generally less than one volt (e.g., 0.6-0.9 volts. Thus, inorder to provide greater output voltage, it is usually necessary tostack numerous cells together and connect electrically in series tocreate a higher voltage fuel cell stack. Fuel cell stacks can then befurther connected in series and/or parallel to form larger arrays fordelivering higher voltages and/or currents.

It is conventional practice to sense or monitor the voltage of variousones, or even all, of the fuel cells in a fuel cell stack assembly inorder to monitor the health, i.e., performance and/or endurancecapacity, of the stack assembly as a whole, or even sub-groups orindividual fuel cells within the stack assembly. Examples of suchvoltage monitoring arrangements are disclosed in U.S. Pat. Nos.5,170,124; 5,763,113; 6,646,418; and 6,724,194, in which voltages sensedat individual ones of the fuel cells in a stack assembly arecollectively analyzed by appropriate processing circuitry.

To provide a voltage signal associated with a particular fuel cell inthe stack assembly, a voltage contact, lead, or pin, herein aftertypically a “lead” or “contact”, is in contact with the particular fuelcell at a discrete location therein or thereon, and acts as a sensor forthat particular fuel cell. While the voltage lead of the sensor may bein contact with any of several locations in a typical fuel cell, onetypical location is contact with the conductive, bipolar plate used toseparate adjacent fuel cells. These separating plates may variously beidentified as flow field plates, separator plates, water transferplates, or the like, and the voltage lead is typically located at ornear the corner of a generally rectangular-shaped fuel cell/separatorplate. A single such lead typically serves to sense the voltage andprovide the associated voltage signal for that particular cell.

While the afore-described arrangements of voltage sensors associatedwith fuel cells in a fuel cell stack assembly are able to revealsignificant information regarding the condition of the cells andassembly, there are debilitating operating modes that these arrangementsof sensors do not detect. Perhaps the most common of these debilitatingmodes is that of local fuel starvation. This condition may destroycathode catalyst; but only in a very local area. A typical voltagemonitor having a voltage lead at one corner of a fuel cell simply cannot differentiate between this type of decay caused by local fuelstarvation, which may be of sudden onset, and the more normal, gradual,and thus manageable decay caused by catalyst recrystallization, carboncorrosion and/or reversible catalyst layer wetting.

What is needed is an arrangement for the effective detection of localproblems in the stack at the level of the individual fuel cell. Anarrangement for the detection of local fuel starvation would be ofparticular benefit.

SUMMARY

The present disclosure recites a fuel cell voltage monitoringarrangement, or scheme, for the effective detection of local problems inan individual fuel cell, and particularly such problems in a fuel cellstack assembly at the level of the individual fuel cell, or a subset offuel cells in the stack assembly. More specifically, the presentdisclosure recognizes that any problem of local character in a stackassembly, herein after a “local stack problem”, results in a shift ofcurrent flow around the problem area and moreover, that such shiftinevitably results in an in-plane current flow in the separator plate.Still further, the disclosure recognizes that that any in-plane currentflow will result in a voltage difference in the plate (and plane).Apparatus and methods are disclosed herein for the utilization of thischaracteristic.

Accordingly, the present disclosure provides two or more spaced voltageleads or contacts in a fuel cell plane, as at a separator plate, todetect the presence of a voltage difference within the plate/plane. Sucha detected voltage difference is thereby indicative of a local stackproblem, as at that particular fuel cell or in the immediate regionthereof. Moreover, such arrangements of two or more voltage contactslocated in a plane may, in one extreme, be applied to each and everyfuel cell in a stack assembly. Alternatively, and more likely andeconomical in the instance of large stacks, the two or more voltagecontacts per fuel cell may be located at intervals “n” of fuel cells inthe stack as, for example, at intervals of every 10, or even every 50,cells. More generally, the two or more voltage contacts or leads percell may be at intervals of n₁, n₂, n₃, . . . n_(x), where each intervalmight be equal or different. In the instance of two in-plane voltagecontacts associated with a particular fuel cell, they will preferably bespaced relatively distantly from each other in the plane of contact, asat two adjacent or at two diagonally opposite, corners of arectilinearly-shaped fuel cell.

As an alternative to monitoring for voltage differentials acrossrespective selected ones of the fuel cells in a fuel cell stack assemblyto detect problems local to the respective selected cell, such a localproblem is detectable in a “regional” manner. A subset of multipleadjacent fuel cells in the stack assembly is provided with at least twovoltage contacts or leads at each of the two fuel cells at the oppositeends of the subset. Each lead of those at least two leads per fuel cellis spaced at the respective fuel cell as described above. However, afirst voltage difference is measured between the pair of leads at a samefirst location on respective fuel cells at opposite ends of the stacksubset, a second voltage difference is measured between the pair ofleads at a same second location on respective fuel cells at oppositeends of the same stack subset, and the first and second voltagedifferences are compared such that a further difference, if any, is anindication of a local problem in, or partly in, that stack subset.

Each and all of the foregoing arrangements is thus able to provide asignal indicative of the operating condition of the fuel cell and/orstack assembly, which signal is then available to a power plantcontroller to effect appropriate response to the condition.

The foregoing features and advantages of the present disclosure willbecome more apparent in light of the following detailed description ofexemplary embodiments thereof as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a fuel cell illustrating the inclusionand spaced location of at least 2 voltage sensing leads in a plane ofthat fuel cell;

FIG. 2 illustrates a schematic block diagram of a fuel cell stackassembly having voltage sensing leads and associated circuitry fordetecting “local” problems in accordance with one aspect of thedisclosure; and

FIG. 3 illustrates a schematic block diagram of a fuel cell stackassembly having voltage sensing leads and associated circuitry fordetecting “local” problems in accordance with another aspect of thedisclosure.

DETAILED DESCRIPTION

Referring to FIG. 1, there is illustrated a perspective view of ageneralized configuration of a fuel cell 10 showing the inclusion of atleast 2 voltage sensing leads 12 a and 12 b positioned in spacedrelation “S” in a plane of that fuel cell. The fuel cell 10 is typicallycomprised of an anode substrate 14 having an anode catalyst 14 a on aface thereof, a cathode substrate 16 having a cathode catalyst 16 a on aface thereof, an electrolyte layer 18 intermediate and contacting thecatalyst-coated surfaces 14 a and 16 a of the anode 14 and cathode 16; acathode separator plate 20 adjacent the outer surface of the cathode 16,and an anode separator plate 22 adjacent the outer surface of the anode14. For the purposes of the present discussion, each of theaforementioned elements or regions of, or within, a fuel cell, to with,the separator plates 20 and 22, the anode 14, the cathode 16 and theelectrolyte (or electrolyte region) 18, are described as being nominallyor generally planar in shape and are arranged in layered assembly. Afurther anode separator plate 22 and anode substrate 14 are illustratedto indicate the manner in which a next-adjacent fuel cell would bepositioned if arrayed in a stack assembly, as is typically the case.

Solely for purposes of discussion herein, the electrolyte 18 will bepresumed to be of the solid polymer membrane type, as in a PEM cell.However, it will be appreciated that the electrolyte might be any of theother various forms, such as phosphoric acid, molten carbonate, etc. andremain within the scope of the disclosure. Moreover, the separatorplates 20 and 22 may take a variety of forms, and are typicallyconductive, bipolar structures that engage the cathode 16 and anode 14,respectively, and typically have grooves or channels to provide flowfields for the fuel, typically H₂, and the oxidant, typically air, aswell as to transfer product water through a porous structure. Moreover,the separator plates 20 and 22 serve to define the structural limits ofan individual fuel cell 10 and to space it from the next adjacent fuelcell in a stack assembly, as well as to provide electrical conductivityand connection there between. It will be understood that although theanode and cathode separator plates are shown and described here asseparate elements or plates, they may in fact be combined as a singlestructure when located intermediate a pair of adjacent fuel cells in astack assembly.

Each of the at least two voltage-sensing contacts, or leads, 12 a and 12b, may be of a variety of known configurations and electrical contactingarrangements with a coplanar portion of the fuel cell 10. As shownherein, it is convenient for each of the voltage leads 12 a and 12 b tobe metal conductors, perhaps of noble metals, that are positioned ingood physical and electrical contact with the interface between theanode separator plate 22 and the anode 14, as by an interference fitwithin a groove or channel 24 provided in the mating face of theseparator plate 22 and/or anode 14. Importantly, the contacts 12 a and12 b are spaced from one another across the surface of the mating faceof the separator plate 22 by a distance “S” illustrated on FIG. 1. Theplan profile of the fuel cell 10, and particularly its constituentelements, is generally rectilinear, and typically rectangular.Accordingly, the contacts 12 a and 12 b are typically located at or neartwo opposite corners of the separator plate 22, though positioning at ornear two adjacent corners may also be suitable, particularly if or asthe rectangular shape approaches that of a square.

Maximizing the distance between the two contacts 12 a and 12 b within aplane of the fuel cell 10 generally increases the opportunity to senseany voltage difference arising in the plane as the result of in-planecurrent flow caused by local problems such as local fuel starvation.Referring to FIG. 1, a region designated 30 is shaded to represent aregion of fuel starvation at least in the area of the interface betweenthe separator plate 22 and the anode 14, though the effects may tend toextend through the fuel cell 10 for some limited distance in thedirection of the next adjacent fuel cells to either side of it in astack assembly. Whereas the normal flow of current is as depicted byarrows “I” toward the left side of FIG. 1, the presence of that localregion 30 of fuel starvation results in a shift of current flow in theproblem area as depicted by the curved arrows “I” near the region 30.This current flow shift necessarily results in an in-plane current flowin that region, i.e., the separator plate 22, which in turn results in avoltage difference in the general plane of that plate 22. Thus, theutilization of two or more voltage sensors 12 a, 12 b, etc. generally inthat plane and spaced from one another will detect that voltagedifference, which is in turn indicative of the existence, and to someextent the magnitude, of the local problem.

Each of the contacts 12 a and 12 b is conductively connected, via linesVa and Vb respectively, to apply the respective sensed voltages to someform of suitable comparative circuitry, as for example comparator 40.The comparator 40 provides a signal or indication 42 of the existence,and perhaps magnitude, of a voltage difference, which is in turnindicative of a local problem in the fuel cell 10 in which the contactsreside. The signal 42 is, or may be, conveniently extended to a fuelcell power plant controller 60 to provide one or more output controlsignals 62 for controlling operation of the fuel cells 10 comprising astack assembly of a power plant.

Referring now to FIG. 2, there is illustrated a schematic block diagramin accordance with one aspect of the disclosure, of a fuel cell stackassembly 110 comprised of a number of fuel cells 10 cascaded in serialelectrical and physical configuration to provide a requisite voltage. Atypical fuel cell stack assembly 110 might range from fewer than 50cells to 250 cells or more. While in the extreme it is theoreticallypossible to locate pairs of voltage contacts at each fuel cell in thestack, the cost and bulk would likely be prohibitive. Instead,significant information on the health of the stack may be obtainedfirstly by placing individual voltage contacts (not separately shown) atvarious intervals along the stack as known from the prior art, but alsoimportantly in accordance with the present disclosure, pairs (or more)of voltage contacts may also be placed at various intervals preferablyalso utilizing some of the individual voltage contacts as one contact ofthe contact pair. The stack assembly 110 is illustrated as having pairsof substantially co-planar voltage contacts, as individually illustratedin FIG. 1, at various intervals, generally designated “n”, along thestack, with the 1^(st) interval being designated “n₁”, the secondinterval being designated “n₂”, the third interval being designated “n₃”and so forth through the last interval that is designated “n_(x)”, where“n” is some integer number and, from interval to interval, may differ orbe the same as one or more or each of the others. Perhaps the mosttypical arrangement is for “n” to be a constant of perhaps ten or twentyor even fifty. Assuming the latter, one might have a stack assembly 110of 250 fuel cells, with individual voltage contacts (not shown)installed at every tenth cell, but with a pair of contacts 12 a and 12 binstalled at each 50^(th) cell, and one contact of that pair being theotherwise individual contact appearing at that location.

As described previously with respect to FIG. 1, the voltage signals Vaand Vb from a respective pair of voltage contacts 12 a and 12 b areextended to appropriate comparison circuitry determining whether or nota voltage differential exists between that pair of contacts and perhapsalso the magnitude of any such difference.

Referring again to FIG. 2, each pair of voltage contacts 12 a and 12 bprovides voltage signals via respective pairs of lines Va₁, Vb₁;Va_(n1), Vb_(n1); Va_(n1+n2), Vb_(n1+n2); throughVa_(n1+n2+n3 . . . +nx), Vb_(n1+n2+n3 . . . +nx), to appropriatecomparator circuitry 140. The voltage comparator circuitry 140 is hereshown as comprised of representative separate comparators 40, forproviding representative signals or indications 42 of the existence andperhaps magnitude of a voltage difference sensed by the respectivecontact pair; however it will be appreciated that such comparisons maybe made by singular circuitry operating to scan the respective pairs oflines in sequence. The various representative voltage differentialsignals 42 from the various comparators 40 may then be extended toprovide respective representative inputs to a voltage differentialthreshold detector 50 for the purpose of providing an output alarm orsignal 52 only if and when a voltage differential of some thresholdmagnitude is present. Alternatively, of course, such threshold could beincorporated in each comparator circuit 40. Still further, it will beunderstood and appreciated that although the analytical voltagecomparator 140 has been illustrated as being comprised of discretecircuits 40 and 50, perhaps the most practical implementation is throughthe use of a suitably programmed digital processor or analyzer in whichthe voltages on lines Va₁, Vb₁, etc., are converted to digital valuesand perhaps then provided as multiplexed inputs to the processor forsequential comparative analysis as various pairs to eventually provideessentially the same information.

Indeed, in keeping with this latter characterization, the output signal52 of the voltage differential threshold detector 50 serves as an inputto a/the fuel cell power plant controller 60, which in turn provides oneor more output control signals 62 for controlling operation of the fuelcell stack assembly 110 via various controlled operating components andsystems (not shown). For instance, one or more operating parameters maybe controlled, such as reactant flow rates, coolant flows, fuel celltemperatures, stack assembly power output, etc. Still further, thethreshold detector and/or the power plant controller 60 of which it maybe a part, may recognize not only a satisfactory operating state inwhich operation continues as usual, or an extreme problem status inwhich it is prudent or necessary to shutdown operation of the stackassembly, but also an intermediate operating state in which a moderateoperating problem is determined and operation is allowed to continuewith some modulation and/or modification of one or more operatingparameters in an effort to return to normal operation.

With the above-described arrangement for detecting voltage gradientsacross the plane of an element, or elements, in an individual fuel cell,it is then possible and reasonable to conclude that a local problem atthat cell and/or nearby cells, such as fuel starvation, exists. Onceidentifying the existence and perhaps magnitude of such problem, it isthen possible to terminate operation and/or take appropriate correctiveor remedial action. The intervals within the stack between voltagecontact pairs will determine both the preciseness with which suchproblems can be both detected and located, and also the cost ofachieving such preciseness.

It is also appropriate to note that although the present descriptionaddresses the use of two (a pair of) substantially co-planar voltagecontacts at an individual fuel cell to identify local problems, the useof 3 or even 4 such co-planar contacts would perhaps further increasedetection accuracy but is generally less preferred because of adisproportionate increase in cost.

In an alternative arrangement for detecting a problem of local nature ina fuel cell stack assembly, reference is made to FIG. 3 which is similarin many respects to that of FIG. 2, but which also differs in asignificant manner. Specifically, the fuel cell stack assembly 110 is,or may be, substantially the same as described with respect to FIG. 2,including the placement of pairs (or more) of co-planar voltage contacts12 a and 12 b at various intervals “n” within the stack assembly 110.However, whereas previously the focus was on detecting a voltagedifferential, if any, across the plane of an individual fuel cell, thatfocus now is upon detecting voltage anomalies associated with a subset,or series of subsets, of fuel cells within the stack assembly. This isdone by the use of a pair of co-planar leads (contacts) at one end ofthe stack assembly subset in combination with a pair of co-planar leadsat the other end of the subset. For the sake of convenience and brevityof description and depiction herein, the stack assembly fuel cellsubsets of FIG. 3 comport exactly with the intervals “n” depicted anddescribed as n₁, n₂, n₃ . . . n_(x) with respect to FIG. 2.

In this instance, however, the respective voltages V_(a) and Va_(n1) atthe two (i.e., pair of) leads 12 a that occupy the same position in theplanes of the respective cells FC₁ and FC_(n1+1) at the opposite ends ofthe stack subset n₁ are extended to a voltage comparator ΔVa_(n1)comprising part of an analytical voltage comparator 140 to quantify thevoltage (normally not zero) there between as output 242 a. Similarly,the respective voltages Vb and Vb_(n1) at the two i.e., pair of) leads12 b that occupy the same position, i.e., the “b” position, across therespective cells in the planes of the respective cells FC₁ and FC_(n1+1)at the opposite ends of the stack subset n₁ are extended to a voltagecomparator ΔVb_(n1) to quantify the voltage (normally not zero) therebetween as another output 242 b. Then, importantly, the ΔVa_(n1) and theΔVb_(n1) voltage signals 242 a and 242 b, respectively, are compared atcomparator ΔV_(n1) to provide output 342 _(n1) as the difference, ifany, between the stack subset voltage difference for the “a position” vsthe stack subset voltage difference for the “b position”. In a normalcondition, the voltage difference for the “a” position and for the “b”position for the stack assembly subset should be nominally the same,even though each might be on the order of many volts, depending upon thenumber of fuel cells in the subset. However, if there is a local problemwithin the stack subset as a region, it will tend to manifest itself bymaking the “a” position voltage difference greater than for the “b”position, or vice versa, and the compared output 342 _(n1) will reflectthis.

Accordingly, if an output 342 _(n1), or 342 _(n2), etc, exceeds somenon-zero threshold, it may be taken as an indication of a local problemin that subset. Thus, those outputs 342 _(n1), or 342 _(n2), etc., areextended to a ΔV threshold detector 350 to provide an output signal oralarm 252 if one or more of the subsets indicates a voltage differencesufficient to be of concern. As with FIG. 2, voltage comparatorcircuitry 440 is depicted as being comprised of discrete circuitelements 240, 340, 350, etc, but it will be understood that it may welltake the form of an integral digital processor or analyzer. The outputsignal 252 of the ΔV threshold detector 350 is depicted as serving as aninput to a/the fuel cell power plant controller 60, which in turnprovides one or more output control signals 62 for controlling operationof the fuel cell stack assembly 110 via various controlled operatingcomponents and systems (not shown). In this regard, the functions andcircuitry of the voltage comparator circuitry 440 may be part of adigital processor which also constitutes the fuel cell power plantcontroller 60.

Although the disclosure has been described and illustrated with respectto the exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the spiritand scope of the invention.

1. A fuel cell voltage monitor (12 a, 12 b, 40, 140) for detecting aproblematic operating condition at a fuel cell (10), comprising: a fuelcell (10) having a plurality of nominally planar elements stacked inlayered arrangement, said elements including an anode (14, 14 a), acathode (16, 16 a), an electrolyte (18), and anode and cathode plates(22, 20), the anode (14, 14 a) and cathode (16, 16 a) being on oppositesides of the electrolyte (18) and between the respective plates (22,20); at least two voltage leads (12 a, 12 b) in operative electricalcontact with a same one (eg, 22, 14) of said nominally planar elementsand positioned in spaced apart (S) relation on the plane of the elementto sense the respective voltages thereat; and voltage comparingcircuitry (40, 140) operatively connected (Va, Vb) to the two voltageleads (12 a, 12 b) for comparing the respective voltages and providing asignal (42) indicative of a difference in said respective voltages, asaid voltage difference being occasioned by in-plane current flowcommensurate with the existence of an operating problem at said fuelcell (10).
 2. The fuel cell voltage monitor (12 a, 12 b, 40, 140) ofclaim 1 wherein said anode and cathode plates (22, 20) are separatorplates, and the at least two voltage leads (12 a, 12 b) are in operativeelectrical and physical contact with one of said anode and cathodeseparator plates (22, 20).
 3. The fuel cell voltage monitor (12 a, 12 b,40, 140) of claim 2 wherein said at least two voltage leads (12 a, 12 b)are in operative electrical and physical contact with the anodeseparator plate (22).
 4. The fuel cell voltage monitor (12 a, 12 b, 40,140) of claim 1 wherein the nominally planar elements of the fuel cell(10) are rectilinear and have a plurality of corners, and said at leasttwo voltage leads (12 a, 12 b) are located substantially at differentones of said corners.
 5. The fuel cell voltage monitor (12 a, 12 b, 40,140) of claim 4 wherein said at least two voltage leads (12 a, 12 b) aretwo in number and are located at opposite ones of said corners.
 6. Afuel cell voltage monitor (12 a, 12 b, 40, 140, 440) for detecting aproblematic operating condition at a fuel cell stack assembly (110),comprising: a fuel cell stack assembly (110) comprising a number of fuelcells (10) stacked in serial physical and electrical relation, each fuelcell (10) having a plurality of nominally planar elements stacked inlayered arrangement, said elements including an anode (14, 14 a), acathode (16, 16 a), an electrolyte (18), and anode and cathode separatorplates (22, 20), the anode (14, 14 a) and cathode (16, 16 a) being onopposite sides of the electrolyte (18) and between the respectiveseparator plates (22, 20); at least two voltage leads (12 a, 12 b) inoperative electrical contact with a same one (eg, 22, 14) of saidnominally planar elements and positioned in spaced apart (S) relation onthe plane of the element for each of a plurality (FC₁, FC_(n1+1), . . .FC_(n1+n2+n3 . . . +nx+1)) of said fuel cells (10) at spaced intervals“n” in the stack assembly (110) to sense the respective voltagesthereat; and voltage comparing circuitry (40, 140, 440) operativelyconnected (Va, Vb; Va₁, Vb₁; Va_(n1), Vbn1; Va_(n1+n2), Vb_(n1+n2);Va_(n1+n2+n3 . . . +nx), Vb_(n1+n2+n3 . . . +nx),) to the at least twovoltage leads (12 a, 12 b) in said plurality of said fuel cells (10) inthe stack (110) for comparing the respective voltages and providing asignal (42, 52; 342 n, 352) indicative of a difference in selected onesof said respective compared voltages, a said voltage difference in saidselected ones of said compared voltages being occasioned by in-planecurrent flow commensurate with the existence of an operating problemsubstantially at or between the respective ones of said plurality ofsaid fuel cells (10) in the stack assembly (110) responsible for saidvoltage difference in said selected ones of compared voltages.
 7. Thefuel cell voltage monitor (12 a, 12 b, 40, 140, 440) of claim 6 whereineach of said fuel cell intervals “n” in the stack assembly (110) is thesame.
 8. The fuel cell voltage monitor (12 a, 12 b, 40, 140) of claim 6wherein the voltage comparing circuitry (40, 140) provides signals (42)indicative of a difference in voltage for each of said at least twovoltage leads (12 a, 12 b) for each of said fuel cells (10) at whichsaid at least two voltage leads are located.
 9. The fuel cell voltagemonitor (12 a, 12 b, 40, 140) of claim 8 wherein the voltage comparingcircuitry (40, 140) further includes threshold circuitry (50)operatively connected to the signals (42) indicative of voltagedifferences, if any, for each of said fuel cells (10) at which said atleast two voltage leads are located for indicating (52) that a saidvoltage difference exceeds a predetermined threshold level.
 10. The fuelcell voltage monitor (12 a, 12 b, 40, 440) of claim 6 wherein the atleast two voltage leads (12 a, 12 b) are located at respective first (a)and second (b) spaced positions on the respective fuel cell elementshaving the at least two voltage leads (12 a, 12 b) positioned thereon,said first (a) and said second (b) spaced positions being substantiallythe same for each of the plurality of fuel cell elements having the atleast two voltage leads (12 a, 12 b) positioned thereon, the leads (12a) at a plurality of said first spaced positions (a) being operativelyconnected as pairs to voltage comparing circuitry (ΔVa) to determine andprovide a voltage difference signal (242 a) between the respective pairsof said leads (12 a) at said first positions defining there between arespective subset (n) of the stack assembly, the leads (12 b) at aplurality of said second spaced positions (b) being operativelyconnected to voltage comparing circuitry (ΔVb) to determine and providea voltage difference signal (242 b) between the respective pairs of saidleads (12 b) at said second positions and defining there between thesame said subset (n) of the stack assembly as for said pairs of leads atsaid first position, said first position and said second positionvoltage difference signals (242 a, 242 b) being connected to a voltagecomparator (340) for comparing the respective voltage differences andproviding a further difference signal 342 n indicative of in-planecurrent flow commensurate with the existence of an operating problemsubstantially at or between the respective pair of said fuel cells (10)defining a respective said subset (n) in the stack assembly (110). 11.The fuel cell voltage monitor (12 a, 12 b, 40, 440) of claim 6 furtherincluding a controller (60) to regulate operation of the fuel cell stackassembly 110, the controller being connected and responsive to adifference signal (42, 52; 342 n, 352) to provide a fuel cell stackassembly operating control signal
 62. 12. A method for monitoring andoperating a fuel cell stack assembly (110), the fuel cell stack assemblycomprising a number of fuel cells (10) stacked in serial physical andelectrical relation, each fuel cell (10) having a plurality of nominallyplanar elements stacked in layered arrangement, said elements includingan anode (14, 14 a), a cathode (16, 16 a), an electrolyte (18), andanode and cathode separator plates (22, 20), the anode (14, 14 a) andcathode (16, 16 a) being on opposite sides of the electrolyte (18) andbetween the respective separator plates (22, 20), comprising the stepsof: detecting the voltage at two or more spaced locations (12 a, 12 b)at the same one (eg, 22, 14) of the nominally planar elements in each ofa plurality (FC₁, FC_(n1+1), . . . FC_(n1+n2+n3 . . . +nx+1)) of thefuel cells (10) at intervals “n” in the stack assembly (110) to providerespective voltage signals; comparing (40, 140, 440) two or more of saidvoltage signals (Va, Vb; Va₁, Vb₁; Va_(n1), Vbn1; Va_(n1+n2),Vb_(n1+n2); Va_(n1+n2+n3 . . . +nx), Vb_(n1+n2+n3 . . . +nx),) for eachof two fuel cells at respective opposite ends of a said interval “n” toprovide respective voltage differential signals (42, 52; 342 n, 352);and providing from the respective voltage differential signals (42, 52;342 n, 352), a control signal (62) to regulate operating performance ofthe fuel cell stack assembly.
 13. The method of claim 12 wherein thestep of comparing detected voltages comprises comparing with one anotherthe voltages (Va, Vb; Va_(n1), Vb_(n1); Va_(n1+n2), Vb_(n1+n2)) for theat least two spaced locations (12 a, 12 b) associated with the same fuelcell (10).
 14. The method of claim 12 wherein the step of detectingvoltages comprises: detecting the voltages (Va, Vb, Va_(n1), Vb_(n1),etc) at first and second spaced positions that have the same locations(12 b, 12 a) on each of the fuel cells (10) at said intervals “n”;comparing the detected voltages (Va, Va_(n1); Va_(n1), Va_(n1+n2); etc)for the same first position at each of the two fuel cells (FC₁,FC_(n1+1); etc) at opposite ends of a said interval “n” to provide afirst position voltage differential signal (242 a); comparing thedetected voltages (Vb, Vb_(n1); Vb_(n1), Vb_(n1+n2); etc) for the samesecond position at each of the two fuel cells (FC₁, FC_(n1+1); etc) atopposite ends of a said interval “n” to provide a second positionvoltage differential signal (242 b); comparing the first positionvoltage differential signal (242 a) and the second position voltagedifferential signal (242 b) to provide a resultant voltage differentialsignal (342 n); and providing (350, 60) from the resultant voltagedifferential signal (342 n), the control signal (62) to regulateoperating performance of the fuel cell stack assembly (110).